Upgrade from Confluence 2.5 to 3.0 attachment migration fails if custom space logos present

When migrating from Confluence 2.5 to 3.0, the attachment structure migration task fails if there are custom space logos present.

To reproduce:

1. Create a Confluence 2.5 instance, and upload a custom space logo (and possibly at least one other attachment.)
2. Replace Confluence 2.5 with 3.0.
3. New attachments (in the Confluence 3.0 format) are created by the system for the space logos.
4. The migration of the attachments tree to the new format fails because the 'ver003' directory exists.

Workaround (for UNIX-like environments):

After the above failure case, shut down Confluence and run the following sh command in the attachments directory to de-migrate space logos; the migration will then run properly:

$ for i in ver003/*/*/*/*/*/*; do mv $i .; done
$ rm -rf ver003

(I don't know when the change from space logos being special to being attachments happened. This issue should affect any upgrades from older versions than that to current Confluence releases.)